Traditionally, wireless networks are confined to certain limited parts of the radio spectrum allocated for that purpose. Allocation of the radio spectrum is controlled by nationally regulatory bodies which allocate the radio spectrum according to need, or to licensed users. For example, television broadcasts and mobile communications networks are restricted to specific allocated frequencies. However, it has been found that many frequencies may not be used all of the time, leading to inefficient use of portions of the radio spectrum. If such temporarily un-used portions of the spectrum could be re-used for another purpose, more efficient use could be made of the radio spectrum.
Cognitive radio has been proposed as a way of increasing the efficiency with which the radio spectrum is used. In a cognitive radio network, nodes may communicate using frequencies allocated for a different purpose or to a licensee, but which are not currently in use, for example on a broadcast frequency that is not currently in use. However, in order to ensure that the primary user of the frequency is not inconvenienced by this frequency re-use, secondary users (i.e. the cognitive radios) must detect any signal transmitted by the primary user and immediately cease use of the frequency in order to avoid causing any interference to the primary user.